In the Bayer process for Bauxite ore beneficiation, crystallization and precipitation of solubilized alumina trihydrate values from caustic liquors, referred to herein as Bayer process liquor, is a critical step towards the recovery of aluminum values. Bayer process operators optimize their precipitation methods so as to produce the greatest possible yield from the Bayer process liquors. Production is often limited by processing conditions under which the crystallization and precipitation is conducted. These processing conditions vary from one plant to the next and include temperature profiles, seed charge, seed crystal surface area, liquor loading, liquor purity, and the like.
To obtain the maximum recovery of aluminum values from Bayer process liquor, a plant will typically attempt to maximize the liquor caustic level and minimize the final precipitation temperature. However, it is well known that sodium oxalate can co-precipitate with alumina trihydrate from Bayer process liquor, and that the solubility of sodium oxalate in Bayer process liquor is generally decreased by increasing caustic levels and decreasing precipitation temperatures. Uncontrolled co-precipitation of oxalate can cause a number of problems for a Bayer plant, which place restrictions on plant operating parameters and therefore on aluminum values recovery.
Well known adverse effects of co-precipitation of sodium oxalate include increasing product fines and soda levels, and interference with the efficient size classification of alumina trihydrate. High quality alumina trihydrate usually has a crystal coarseness specification of 90-95% of the crystals being at least 44 microns in diameter. It is known in the art that co-precipitating sodium oxalate can cause the alumina trihydrate crystals to precipitate as a very fine material which is below the usual quality specification of 90-95% of the crystals having 44 microns or greater in diameter.
Alumina trihydrate is precipitated from Bayer process liquor, which in essence is a concentrated sodium hydroxide solution. Typical soda levels in alumina levels are only 0.2-0.6% by weight as Na.sub.2 O. Nonetheless, it is desirable to produce alumina with soda levels at the lower end of this typical range. Modern smelters generally calculate their break-even point for soda in alumina at 0.35% Na.sub.2 O. As smelter technology improves and as older smelters are replaced, the incentive will become greater for alumina producers to meet this low soda level. Furthermore, some markets (e.g. catalysts, catalyst supports, refractories, electroceramics) require even lower soda levels. It would be advantageous to provide a method and composition for reducing soda levels in alumina.
The present invention provides a method for stabilizing solubilized sodium oxalate in a Bayer process liquor. This stabilization effect allows the Bayer process operator to add more caustic (NaOH) and/or lower final precipitation temperatures to improve recovery of aluminum values without adversely affecting product sizing or soda.